domain: add implicit USB controller

Some tools, such as virt-manager, prefers having the default USB
controller explicit in the XML document. This patch makes sure there
is one. With this patch, it is now possible to switch from USB1 to
USB2 from the release 0.9.1 of virt-manager.

Fix tests to pass with this change.

tests/qemuxml2argvdata/qemuxml2argv-blkiotune.xml

@@ -22,6 +22,7 @@
       <target dev='hda' bus='ide'/>
       <address type='drive' controller='0' bus='0' unit='0'/>
     </disk>
+    <controller type='usb' index='0'/>
     <controller type='ide' index='0'/>
     <memballoon model='virtio'/>
   </devices>